Most of the current display apparatuses are implemented with liquid crystal displays (LCDs). Compared to the cathode ray tube (CRT) display, LCD has smaller size, high mobility, low power consumption, etc. However, in respond to the high demand of thin display apparatus and low power consumption, organic light emitting diode displays have been seen in market in recent years. The main difference between the organic light emitting diode displays and the conventional LCDs is that the organic light emitting diode displays have the self-luminance feature. Therefore, the organic light emitting diode displays have smaller size, lower manufacturing cost and less power consumption due to no backlight device is required. However, the pixel units in the organic light emitting diode display may have aging issue which is resulted by the repeating displaying operations, and consequentially the organic light emitting diode display may have incorrect display issues. Thus, it is quite important to develop a mechanism to effectively detect the aging of the pixel unit effectively and accurately compensate the aging in time.